Vadose Zone: The zone between land surface and the water table
within which the moisture content is less than saturation (except in the
capillary fringe) and pressure is less than atmospheric. Soil pore space
also typically contains air or other gases. The capillary fringe is included
in the vadose zone. (See: Unsaturated Zone.)

Valued Environmental Attributes/Components: Those aspects(components/processes/functions)
of ecosystems, human health, and environmental welfare considered to be
important and potentially at risk from human activity or natural hazards.
Similar to the term "valued environmental components" used in environmental
impact assessment.

Vapor: The gas given off by substances that are solids or liquids at ordinary atmospheric pressure and temperatures.

Vapor Capture System: Any combination of hoods and ventilation
system that captures or contains organic vapors so they may be directed
to an abatement or recovery device.

Vapor Dispersion: The movement of vapor clouds in air due to wind,
thermal action, gravity spreading, and mixing.

Vapor Plumes: Flue gases visible because they contain water droplets.

Vapor Pressure: A measure of a substance's propensity to evaporate,
vapor pressure is the force per unit area exerted by vapor in an equilibrium
state with surroundings at a given pressure. It increases exponentially
with an increase in temperature. A relative measure of chemical volatility,
vapor pressure is used to calculate water partition coefficients and volatilization
rate constants.

Vapor Recovery System: A system by which the volatile gases from gasoline are captured instead of being released into the atmosphere.

Variance: Government permission for a delay or exception in the
application of a given law, ordinance, or regulation.

Vector: 1. An organism, often an insect or rodent, that carries
disease. 2. Plasmids, viruses, or bacteria used to transport genes into
a host cell. A gene is placed in the vector; the vector then "infects"
the bacterium.

Vehicle Miles Travelled (VMT): A measure of the extent of motor
vehicle operation; the total number of vehicle miles travelled within
a specific geographic area over a given period of time.

Ventilation Rate: The rate at which indoor air enters and leaves
a building. Expressed as the number of changes of outdoor air per unit
of time (air changes per hour (ACH), or the rate at which a volume of
outdoor air enters in cubic feet per minute (CFM).

Ventilation/Suction: The act of admitting fresh air into a space
in order to replace stale or contaminated air; achieved by blowing air
into the space. Similarly, suction represents the admission of fresh air
into an interior space by lowering the pressure outside of the space,
thereby drawing the contaminated air outward.

Venturi Scrubbers: Air pollution control devices that use water
to remove particulate matter from emissions.

Vinyl Chloride: A chemical compound, used in producing some plastics,
that is believed to be oncogenic.

Virgin Materials: Resources extracted from nature in their raw
form, such as timber or metal ore.

Viscosity: The molecular friction within a fluid that produces
flow resistance.

Volume Reduction: Processing waste materials to decrease the amount
of space they occupy, usually by compacting, shredding, incineration,
or composting.

Volumetric Tank Test: One of several tests to determine the physical
integrity of a storage tank; the volume of fluid in the tank is measured
directly or calculated from product-level changes. A marked drop in volume
indicates a leak.

Vulnerability Analysis: Assessment of elements in the community
that are susceptible to damage if hazardous materials are released.

Vulnerable Zone: An area over which the airborne concentration
of a chemical accidentally released could reach the level of concern.